Optical communication systems use pulses of light to transmit data. The light used to create these pulses necessarily contains a band of wavelengths, rather than a single wavelength, even though the light is generally from laser sources. A broadening of the wavelength band occurs when the light is pulsed, due to conditions imposed by the Fourier transform spectrum of the pulse shape. Some devices, such as electro-absorption modulators (EAM's), used to generate the pulses broaden the wavelength band of the light even further than the transform limit. The desire for ever higher speed optical communications requires ever shorter pulses of light, with ever larger wavelength bands. A problem related to this growing wavelength band of optical pulses is temporal broadening due to chromatic dispersion.
Many components of optical communications system, such as the optical fibers, EAM's, semiconductor optical amplifiers (SOA's), and variable optical attenuators (VOA's), have chromatic dispersion. The existence of chromatic dispersion in the system means that different wavelengths of light travel at different speeds within the system. This phenomenon may lead to temporal pulse broadening. For example, in SMF28 fiber, operating at 1550 nm, longer wavelengths of light travel more slowly within the optical fiber than shorter wavelengths. The dispersion factor, D, in this example is 17 ps/nm/km. Therefore, the shorter wavelengths in a pulse move ahead of the longer wavelengths, broadening the pulse. As the length of the fiber increases this broadening accumulates and chromatic dispersion ultimately limits the maximum distance a data stream can be transmitted before the pulses become indistinguishable due to broadening. For a transform-limited pulse operating at 40GB/S, the distance before the signal deteriorates significantly due to chromatic dispersion is approximately 4 km.
It is, therefore, desirable to compensate for this chromatic dispersion effect by introducing dispersion compensators, which have an opposite effect. A number of methods for introducing this dispersion compensation have been used, such as dispersion compensating fibers (DCF's) or fiber Bragg gratings. Generally, these methods are not easily tunable and, therefore, can only be used for compensating fixed dispersion values. It is impractical to design a specific DCF or fiber Bragg grating for every possible chromatic dispersion due to various lengths of optical fiber or other optical components in a communications signal. Also, in wavelength division multiplex systems, owing to differences in the slope of dispersion compensated fibers and standard fibers such as SMF28 fibers, each channel would need to be fine tuned individually.
In a reconfigurable optical communications network the chromatic dispersion may be varied when the path length changes due to cable damage, overload, or other rerouting. This also may be a problem for fixed dispersion compensation devices such as DCF's or fiber Bragg gratings.
U.S. Pat. No. 6,363,187 B1 to Fells et al. describes an optical waveguide provided with a linearly chirped Bragg reflective grating which can be used to provide linear dispersion compensation. The amount of dispersion compensation provided by this device can be adjusted by changing the magnitude of the axial strain imposed on the grating. Fells et al. disclose that, adjusting the linear dispersion of the grating in this manner requires the presence of a quadratic chirp term, either within the grating itself or within the strain placed upon the grating. This quadratic chirp term leads to additional difficulties and, therefore, should be compensated, at least in part, by causing the optical signal to be reflected by a second Bragg reflective grating with a quadratic component of chirp having an opposite sign to that of the original Bragg reflective grating.
Fells et al. also note that similar effects may be achieved by adjusting the effective refractive index of the waveguide grating structure by changing the temperature of the waveguide, possibly creating a temperature gradient along the longitudinal access of the waveguide in the process. In this temperature tunable dispersion compensating device disclosed by Fells et al., the temperature in both the original chirped Bragg reflective grating and the secondary Bragg reflective grating used for quadratic chirp term compensation should be varied simultaneously.
There is therefore a useful role for an adjustable amplitude linear dispersion compensation device. Such a device could be one designed for operation on its own to achieve substantially complete dispersion compensation. Alternatively, it could be one designed for operation in association with a fixed amplitude dispersion compensation device, such as a length of DCF, that provides a level of compensation which is inadequately matched on its own. The adjustable device may be operated with some form of feedback control loop to provide active compensation that can respond to dynamic changes of dispersion within the system, and in suitable circumstances to step changes resulting from rerouting occasioned, for instance, by a partial failure of the system such as a transmission fiber break.